Many synthetic resin materials are now available for engineering applications and are widely used because of their particular advantages. Using such materials, products can be mass produced at relatively low cost for many uses, and the products are comparatively light weight, corrosion resistant, and can be designed to have very high strength. The structural characteristics of synthetic resin are enhanced by reinforcing with fibers, as in chopped, tow, roving, fabric or tape form, when forming composite or fiber reinforced plastic (FRP) bodies. Where the product shape needed is a closed surface, such as a tubular shape or a vessel, and when high force loading requirements must also be met, continuous filament winding is used. Continuous filaments of high tensile strength wound in predetermined patterns can achieve strength and stiffness properties along with strength to weight ratios that exceed conventional structural materials.
The advantages of FRP materials have not, however, been fully realized where threaded connections are employed because of both production and stress distribution problems. Use of the potential properties of filament wound reinforcement has often been limited by the fact that the geometrical discontinuities at the threads prevent optimum relationships from being achieved between filament orientation and the desired ratio of filament to resin matrix. Furthermore, while a female member can be wound on a male mold having a properly placed thread profile, fiber reinforced threads cannot be molded into the outside surface of a male member with comparable results. Instead, the thread profile must be cut into the outer surface, weakening thread strength and also creating surface irregularities which are incompatible with molded female thread surfaces.
There are a great many potential applications for FRP tubular products, which have in fact been used under specialized conditions in various industries for more than the past 40 years. Composites of these kinds have been used to a limited extent in the chemical and storage cavern industries, and to a greater but still minor extent in the pipeline industry, where usage has been in the range of 11% to 13% of total pipe consumed. The barriers have been the inability to contain internal pressures of the range of 3,000 psi and above, and premature degradation of the threads on repeated makeups and breakouts. The same factors have restricted use of composite pipe in drilled oil, gas and injection wells to a level of 3% to 5% of total pipe consumed. Where drilling is to be done, FRP pipe with sufficiently robust connections can be used if the force loading is light, as in workover drilling.
To obtain proper sealing against high internal pressures (e.g. greater than 3,000 psi), male and female threaded members must be engaged with a high level of torque to achieve high surface bearing pressure. In the prior art, however, this deforms the FRP pipe such that it either cracks or internally "leaks between layers" (delaminates). In addition, where the male threads have been cut into the FRP part, they deny the benefits of the reinforcement and leave filament ends flush at the surface. With wear or high bearing pressure these filament ends form highly frictional cutting points. On makeup and breakout the threads are quickly worn and galled, and breakout becomes increasingly more difficult.
In consequence, when FRP pipe is used, the makeup torque for a given pipe has been confined only to a low range (280-300 ft. lbs. for 2-7/8" pipe, for example). This means that only a comparably low level of bearing pressure can be established when makeup is completed, and thus only low internal pressures, temperature cycles, and tensile loads can be withstood. Consequently, those many industries which can potentially use FRP tubular goods have not been able to rely on them except for low pressure and low duty cycle situations. This includes not only drilled well and down hole applications and workover situations, but also secondary recovery uses, the deep well disposal industry for hazardous materials and pipeline applications.
In the oil industry, defined physical standards for tubular goods for down-hole and horizontal applications have been observed for many years, primarily with respect to threaded steel pipe. Thread profiles, material grades and thicknesses, tapers and other properties have been established for threaded tubular goods by the American Petroleum Institute (API) in accordance with the tensile forces and pressures that must be met for a particular installation. The female member is usually referred to as the "box" and the male member is usually referred to as the "pin". With many API connections, the box is an exterior collar for receiving pins from opposite ends. In oil field parlance, smaller diameter tubular goods are referred to as "tubing" and larger diameter down-hole tubular goods are referred to as "casings".
In assembly of a pipe string for a conventional down-hole installation a length (typically 30 feet, about 9 meters) of a tubing is added (stabbed) into the open upper end of a collar on the upper end of the immediately prior pipe length. The new pipe is threaded into place, until a predetermined range of thread interference, and consequently surface bearing pressure, has been reached. This range is generally established in practice by rotating the newly added pipe length for a predetermined number of turns after initial firm contact (the so-called "hand-tight plane") or until a measured or estimated level of makeup torque has been applied. As more torque is applied, the box is increasingly stressed circumferentially, while the pin is compressed. The amount of deformation locally varies because of relative differences in wall thickness along the length of the threaded region. However, as stress is increased, the deformations tend to increase as well (although not precisely linearly) along the threaded length, as long as the stresses are within the elastic limit of the materials. Where one member ends and the other is continuous, however, the stress level variations become significantly non-linear. In FRP tubular products, these variations are more than twice as steep as those observed with steel.
In view of the fact that male threads in an FRP member are not fully compatible with molded and reinforced female threads, workers in the art have used double ended steel pins or nipples to interconnect two abutting and internally threaded boxes. In this configuration, the deformation is taken up essentially entirely in the FRP member, which therefore must absorb the forces of make-up torque before failure. Existing connections of this type fail at levels which compare very unfavorably with steel tubing connections, in which the torque that can be applied is orders of magnitude greater. The industry has been searching for configurations which will accept substantially higher makeup torques and which will, in the field, indefinitely withstand internal pressures of 5,000 psi and greater. It is known from finite element analyses for steel, and observed failure patterns, that the stresses are highest at the region adjacent to the end of the box, because in this region the threads of the box are most often destroyed or the box itself tends to rupture or delaminate. Resolutions for those problems have not heretofore been found.